Connect the device and open iTunes. In the source list (the left column of iTunes), double-click the device name to type a new name for the device, and then press Return on the keyboard. Synchronize the device (press "Synchronize" in the lower right corner of the screen).
Step 2 use iOS devices
On the device, tap settings > general > about this MAC. Click on the first line where the device name should be displayed. Rename the device using the onscreen keyboard, then tap Done.